US20180224335A1 - Laser frequency measurement method using optical frequency comb - Google Patents
Laser frequency measurement method using optical frequency comb Download PDFInfo
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- US20180224335A1 US20180224335A1 US15/943,429 US201815943429A US2018224335A1 US 20180224335 A1 US20180224335 A1 US 20180224335A1 US 201815943429 A US201815943429 A US 201815943429A US 2018224335 A1 US2018224335 A1 US 2018224335A1
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- 230000003287 optical effect Effects 0.000 title claims abstract description 48
- 238000000691 measurement method Methods 0.000 title claims description 10
- 238000005259 measurement Methods 0.000 claims abstract description 30
- 230000006641 stabilisation Effects 0.000 claims description 15
- 238000011105 stabilization Methods 0.000 claims description 15
- 238000000034 method Methods 0.000 claims description 7
- 230000000087 stabilizing effect Effects 0.000 claims description 7
- 230000035559 beat frequency Effects 0.000 description 29
- 230000010355 oscillation Effects 0.000 description 10
- 238000001228 spectrum Methods 0.000 description 10
- 239000003381 stabilizer Substances 0.000 description 6
- 239000013307 optical fiber Substances 0.000 description 5
- 238000010586 diagram Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 3
- 230000001360 synchronised effect Effects 0.000 description 3
- 238000007796 conventional method Methods 0.000 description 2
- 230000005284 excitation Effects 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 1
- 230000003321 amplification Effects 0.000 description 1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/04—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by beating two waves of a same source but of different frequency and measuring the phase shift of the lower frequency obtained
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/1305—Feedback control systems
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2203/00—Function characteristic
- G02F2203/54—Optical pulse train (comb) synthesizer
Definitions
- the present invention relates to a laser frequency measurement method and device using an optical frequency comb. More specifically, the present invention relates to a laser frequency measurement method and device using an optical frequency comb that can measure the frequency of a laser having large frequency variation and low stability.
- Patent Literature 1 Japanese Patent Application Laid-Open No. 2007-256365
- This optical frequency comb device is a device that outputs a laser having a comb-shaped spectrum with a repetition frequency (longitudinal mode spacing) of f rep , and has the property of having the precisely equal f rep in any frequency band.
- Non-Patent Literature 1 Matsumoto, M. Wouters, B. Warrington, and N. Brown, “Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633 nm,” IEEE Transactions on Instrumentation and Measurement, vol. 58, pp. 1234-1240, Apr. 2009, which is hereinafter called Non-Patent Literature 1.
- FIG. 1 shows the frequency spectra of an optical frequency comb (also called optical comb) and a laser to be measured.
- An oscillation frequency v n in the n-th comb mode of the optical comb can be represented by the following formula:
- f CEO represents a carrier envelope offset frequency (hereinafter called CEO frequency)
- n represents a mode order that indicates the number of an order of a mode with setting an initial mode as the 0-th mode.
- the frequency v laser can be obtained by the following formula (3) using the formulas (1) and (2):
- the repetition frequency f rep and the CEO frequency f CEO of the optical frequency comb are synchronized with a standard frequency (for example, a frequency synchronized with coordinated universal time) and the beam frequency f B is measured, it is possible to accurately measure (calculate) the absolute frequency v laser of the laser to be measured by setting the appropriate integer “n”.
- FIG. 2 is a schematic view of a spectrum of a beat frequency signal that occurs when a photodetector receives the interference between the optical frequency comb and the laser to be measured.
- a harmonic occurs repeatedly at every frequency of f rep . Therefore, in order to measure the beat frequency f B with high accuracy, it is required to cut off unnecessary frequency components other than the beat frequency f B by using a band pass filter (BPF) for an RF frequency signal.
- BPF band pass filter
- the band pass filter extracts frequency signal in a frequency band that depends on specifications of each part.
- a band pass filter for extracting a frequency component of 30 MHz generally passes frequency signal having a frequency of the order of 30 MHz ⁇ 3 MHz. In this case, if the beat frequency f B to be measured varies by 3 MHz or more, the beat frequency f B is cut off by the band pass filter and cannot be measured.
- the optical frequency comb device is just a device specific to measurement of the frequency of such a laser of great accuracy that is stabilized in a molecule absorption line.
- the present invention has been made in order to solve the above-described problem in the conventional technique, and an object thereof is to measure the frequency of a laser having large frequency variation and low stability, which is hard to measure in the conventional technique.
- a laser frequency measurement method using an optical frequency comb in which the frequency of a laser is measured by measuring a frequency of a beat signal generated by the interference between the optical frequency comb, used as the reference of measurement, and the laser to be measured, at least one of a repetition frequency f rep and a CEO frequency f CEO of the optical frequency comb is changed such that the frequency of the beat signal becomes a predetermined value, and the frequency of the laser is measured by measuring the frequency of the beat signal.
- the repetition frequency f rep and the CEO frequency f CEO may be measured, and at least one of the repetition frequency f rep and the CEO frequency f CEO may be changed such that the frequency of the beat signal becomes the predetermined value.
- an absolute frequency v laser of the laser may be measured from measurement values of the repetition frequency f rep and the CEO frequency f CEO , and the predetermined value or measured value of the beat signal.
- the repetition frequency f rep may be stabilized by phase synchronization with an f rep reference frequency, which is generated for stabilization of the repetition frequency f rep
- the CEO frequency f CEO may be stabilized by phase synchronization with an f CEO reference frequency, which is generated for stabilization of the CEO frequency f CEO
- at least one of the f rep reference frequency and the f CEO reference frequency may be changed such that the frequency of the beat signal becomes the predetermined value.
- the repetition frequency f rep may be stabilized by phase synchronization with the f rep reference frequency, which is generated for stabilization of the repetition frequency f rep , and the CEO frequency f CEO may be measured and changed such that the frequency of the beat signal becomes the predetermined value to measure the oscillation frequency of the laser.
- the reference frequencies may be generated by using a frequency synthesizer.
- the present invention provides a laser frequency measurement device using an optical frequency comb, in which the frequency of a laser is measured by measuring a frequency of a beat signal generated by the interference between the optical frequency comb, used as the reference of measurement, and the laser to be measured.
- the laser frequency measurement device includes means for changing at least one of a repetition frequency f rep and a CEO frequency f CEO of the optical frequency comb such that the frequency of the beat signal becomes a predetermined value, and means for measuring the frequency of the beat signal to measure the frequency of the laser.
- the laser frequency measurement device may include means for measuring the repetition frequency f rep , means for measuring the CEO frequency f CEO , and means for changing at least one of the repetition frequency f rep and the CEO frequency f CEO such that the frequency of the beat signal becomes the predetermined value.
- the laser frequency measurement device may include means for measuring an absolute frequency v laser of the laser from measurement values of the repetition frequency f rep and the CEO frequency f CEO , and the predetermined value or measured value of the beat signal.
- the laser frequency measurement device may include means for stabilizing the repetition frequency f rep by phase synchronization with an f rep reference frequency, which is generated for stabilization of the repetition frequency f rep , means for stabilizing the CEO frequency f CEO by phase synchronization with an f CEO reference frequency, which is generated for stabilization of the CEO frequency f CEO , and means for changing at least one of the f rep reference frequency and the f CEO reference frequency such that the frequency of the beat signal becomes the predetermined value.
- the laser frequency measurement device may include means for stabilizing the repetition frequency f rep by phase synchronization with the f rep reference frequency, which is generated for stabilization of the repetition frequency f rep , means for measuring the CEO frequency f CEO , and means for changing the CEO frequency f CEO such that the frequency of the beat signal becomes the predetermined value.
- the laser frequency measurement device may include means for generating the reference frequencies by using a frequency synthesizer.
- stabilization control is performed by changing one or both of the repetition frequency f rep and the CEO frequency f CEO of the optical frequency comb in the direction of compensating a variation of the beat frequency. Therefore, it is possible to measure the frequency of a laser that varies in a wide range, irrespective of limitations of a band of a band pass filter, which is used in measurement of the beat frequency. This configuration can achieve the measurement of the frequency of an inexpensive stable laser, which is often used in an industrial field.
- FIG. 1 is a graph showing examples of spectra of an optical frequency comb and a laser
- FIG. 2 is a graph showing an example of a spectrum of a beat frequency signal
- FIG. 3 is a block diagram showing the configuration of a first embodiment of the present invention.
- FIG. 4 is a drawing of an example of the optical frequency comb used in the first embodiment
- FIGS. 5A to 5C are graphs showing a spectrum of the beat frequency signal after passing through a band pass filter in the first embodiment
- FIG. 6 is a block diagram showing the configuration of a second embodiment of the present invention.
- FIG. 7 is a block diagram showing the configuration of a third embodiment of the present invention.
- FIG. 8 is a block diagram showing the configuration of a fourth embodiment of the present invention.
- FIG. 3 shows a first embodiment of a laser frequency measurement device using an optical frequency comb according to the present invention.
- An optical frequency comb 100 is stabilized by an f rep control signal and an f CEO control signal from a controller 110 .
- a frequency counter 120 measures a stabilized repetition frequency f rep
- a frequency counter 130 measures a stabilized CEO frequency f CEO .
- a personal computer (PC) 140 receives measurement values thereof, and monitors the oscillation frequency of the optical frequency comb 100 .
- a photodetector 160 detects a beat signal generated by a laser 150 to be measured and the optical frequency comb 100 .
- a frequency counter 170 measures the frequency of the beat signal, and the PC 140 receives a measurement value thereof.
- a frequency change command is sent to the controller 110 , so that the controller 110 sends the f rep control signal and/or the f CEO control signal to the optical frequency comb 100 .
- a reference numeral 152 refers to a mirror
- a reference numeral 154 refers to a half mirror
- optical frequency comb 100 An example of the optical frequency comb 100 will be described with reference to FIG. 4 , which cites FIG. 1 of Patent Literature 1.
- an optical frequency comb oscillator 10 first, light excited by an LD (laser diode) 12 generates in a ring resonator a laser that supports a plurality of longitudinal modes. Then, by adjusting a plane of polarization of the laser orbiting in the ring resonator with the use of polarizing elements ( 14 , 15 , and 16 ), such as a wave plate and a polarizing plate disposed in the ring resonator, phase synchronization among the plurality of longitudinal modes occurs, and a pulsed laser is generated.
- polarizing elements 14 , 15 , and 16
- the frequency spectrum of the pulsed laser at this time is in the shape of a comb having a repetition frequency of f rep .
- the repetition frequency f rep can vary by varying the length of the resonator.
- the repetition frequency f rep is changed by adjustment of an extension amount of an optical fiber 11 by a PZT 13 .
- the CEO frequency f CEO can vary by varying the excitation power, and hence is controlled by changing an injected current from a driver to the LD 12 .
- a reference numeral 2 refers to a laser light source.
- Reference numerals 17 and 18 each refer to an optical isolator.
- Reference numerals 21 A and 21 B each refer to a ⁇ /4 plate.
- Reference numerals 22 A and 22 B each refer to a ⁇ /2 plate.
- Reference numerals 30 A and 30 B each refer to an optical fiber amplifier.
- Reference numerals 31 A and 31 B each refer to an optical fiber for amplification.
- Reference numerals 32 A and 32 B each refer to an excitation light source.
- Reference numerals 40 A and 40 B each refer to a highly-non-linear optical fiber.
- Reference numerals 41 A and 41 B each refer to a single mode optical fiber.
- Reference numerals 51 A, 51 B and 51 C each refer to a lens.
- a reference numeral 52 A refers to a non-linear optical medium.
- a reference numeral 53 B refers to a mirror.
- Reference numerals 54 A and 54 B each refer to a half mirror.
- a reference numeral 55 A refers to a band pass filter.
- a reference numeral 56 A refers to a CEO frequency detector.
- a reference numeral 56 B refers to a heterodyne detector.
- a reference numeral 56 C refers to a repetition frequency detector.
- a reference numeral 60 refers to a CEO frequency stabilizer.
- a reference numeral 70 refers to a repetition frequency stabilizer.
- the CEO frequency stabilizer 60 and the repetition frequency stabilizer 70 are used for stabilizing the frequencies.
- the CEO frequency stabilizer 60 and the repetition frequency stabilizer 70 are used in an opposite manner to change the frequencies.
- the PC 140 sends the oscillation frequency change command.
- the controller 110 sends the f rep and/or f CEO control signals, to perform frequency control so as to compensate a variation of the beat frequency f B .
- a beat frequency that is generated by the interference between the optical frequency comb and the laser to be measured having the spectra of FIG. 1 as an example, if the beat frequency f B increases due to increase in the frequency v laser , the repetition frequency f rep is increased or the CEO frequency f CEO is increased under control, so that the beat frequency f B is changed to be in the predetermined range, for example, returned to the center of the BPF.
- the state of FIG. 5B is changed to a state of FIG. 5C .
- Measuring the repetition frequency f rep and/or the CEO frequency f CEO changed at this time by the frequency counters 120 and 130 facilitates obtainment of the absolute frequencies of the optical frequency comb 100 , and therefore it is possible to measure the absolute frequency V laser of the laser 150 to be measured without any problems.
- the repetition frequency f rep and/or the CEO frequency f CEO are synchronized in phase to f rep and/or f CEO reference frequencies, respectively, each of which is generated in accordance with the repetition frequency f rep and/or the CEO frequency f CEO .
- a frequency synthesizer is used.
- An input of a highly accurate standard frequency of 10 MHz makes it possible to generate the reference frequencies with great accuracy.
- the optical frequency comb that is stabilized by phase synchronization with the reference frequencies constitutes the laser frequency measurement device with high accuracy.
- FIG. 6 shows a second embodiment of the present invention for realizing this method.
- the PC 140 Upon receiving the measurement value of the beat frequency signal, the PC 140 sends a reference frequency change command to frequency synthesizers 200 and/or 210 , which generate the f rep or f CEO reference frequencies respectively, so that the beat frequency is controlled to fall within the predetermined frequency range. How to change the repetition frequency f rep and/or the CEO frequency f CEO in accordance with an actual change of the beat frequency f B is the same as that of the first embodiment.
- the accuracy of frequency stabilization by the phase synchronization is much higher than the accuracy of frequency measurement by the frequency counters, it is possible to perform frequency measurement with higher accuracy than the first embodiment.
- FIG. 7 shows a system configuration that more easily achieves accuracy sufficient for practical use, according to a third embodiment.
- the repetition frequency f rep is stabilized by phase synchronization with the f rep reference frequency having high accuracy, with the use of the frequency synthesizer 200 .
- the CEO frequency f CEO is controlled by sending a command signal from the PC 140 to an LD controller 300 so that the beat frequency f B measured by the frequency counter 170 falls within the predetermined range.
- the frequency counter 130 measures the controlled CEO frequency f CEO . If the beat frequency f B varies due to a variation in the oscillation frequency of the laser 150 , the CEO frequency f CEO is changed so as to compensate a variation of the beat frequency f B .
- a method may be combined in which the PC 140 commands the frequency synthesizer 200 to generate the f rep reference frequency, and to change the f rep reference frequency, so that the beat frequency f B varies.
- the combination allows measurement of a variable laser frequency in a wide range, out of the range from ⁇ f CEO ⁇ f B to + ⁇ f CEO + ⁇ f B .
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Abstract
Description
- The disclosure of Japanese Patent Application No. 2014-141965 filed on Jul. 10, 2014 including specifications, drawings and claims is incorporated herein by reference in its entirety.
- The present invention relates to a laser frequency measurement method and device using an optical frequency comb. More specifically, the present invention relates to a laser frequency measurement method and device using an optical frequency comb that can measure the frequency of a laser having large frequency variation and low stability.
- To measure a laser frequency, a method of using an optical frequency comb device is proposed in recent years. For example, as described in Japanese Patent Application Laid-Open No. 2007-256365 (hereinafter called Patent Literature 1), the use of the optical frequency comb device facilitates measurement of the oscillation frequency of a laser with high accuracy. This optical frequency comb device is a device that outputs a laser having a comb-shaped spectrum with a repetition frequency (longitudinal mode spacing) of frep, and has the property of having the precisely equal frep in any frequency band. (See, for example, H. Inaba, Y. Nakajima, F. L. Hong, K. Minoshima, J. Ishikawa, A. Onae, H. Matsumoto, M. Wouters, B. Warrington, and N. Brown, “Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633 nm,” IEEE Transactions on Instrumentation and Measurement, vol. 58, pp. 1234-1240, Apr. 2009, which is hereinafter called Non-Patent Literature 1.)
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FIG. 1 shows the frequency spectra of an optical frequency comb (also called optical comb) and a laser to be measured. - An oscillation frequency vn in the n-th comb mode of the optical comb can be represented by the following formula:
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v n =n·f rep +f CEO (1) - In the formula, fCEO represents a carrier envelope offset frequency (hereinafter called CEO frequency), and “n” represents a mode order that indicates the number of an order of a mode with setting an initial mode as the 0-th mode.
- Here, by interference between the laser to be measured (having a frequency of vlaser) and the optical comb, a frequency difference fB therebetween is observed as a beat signal, as represented by the following formula (2):
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f B =v laser ·v n (2) - Thus, the frequency vlaser can be obtained by the following formula (3) using the formulas (1) and (2):
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v laser =n·f rep +f CEO +f B (3) - Therefore, if the repetition frequency frep and the CEO frequency fCEO of the optical frequency comb are synchronized with a standard frequency (for example, a frequency synchronized with coordinated universal time) and the beam frequency fB is measured, it is possible to accurately measure (calculate) the absolute frequency vlaser of the laser to be measured by setting the appropriate integer “n”.
-
FIG. 2 is a schematic view of a spectrum of a beat frequency signal that occurs when a photodetector receives the interference between the optical frequency comb and the laser to be measured. The beat frequency signal has peaks at the repetition frequency frep by the interference among the modes of the optical frequency comb itself and a frequency fB, (=frep−fB) conjugate to the beat frequency fB, in addition to a peak at the beat frequency fB to be measured. Moreover, relative to fundamentals of these three frequencies, a harmonic occurs repeatedly at every frequency of frep. Therefore, in order to measure the beat frequency fB with high accuracy, it is required to cut off unnecessary frequency components other than the beat frequency fB by using a band pass filter (BPF) for an RF frequency signal. - The band pass filter extracts frequency signal in a frequency band that depends on specifications of each part. For example, a band pass filter for extracting a frequency component of 30 MHz generally passes frequency signal having a frequency of the order of 30 MHz±3 MHz. In this case, if the beat frequency fB to be measured varies by 3 MHz or more, the beat frequency fB is cut off by the band pass filter and cannot be measured.
- Accordingly, it becomes difficult to measure the frequency of an inexpensive stable laser often used in an industrial field in which the oscillation frequency of the laser is simply stabilized by temperature control or the like. In other words, the optical frequency comb device is just a device specific to measurement of the frequency of such a laser of great accuracy that is stabilized in a molecule absorption line.
- The present invention has been made in order to solve the above-described problem in the conventional technique, and an object thereof is to measure the frequency of a laser having large frequency variation and low stability, which is hard to measure in the conventional technique.
- To solve the above problem, according to the present invention, in a laser frequency measurement method using an optical frequency comb, in which the frequency of a laser is measured by measuring a frequency of a beat signal generated by the interference between the optical frequency comb, used as the reference of measurement, and the laser to be measured, at least one of a repetition frequency frep and a CEO frequency fCEO of the optical frequency comb is changed such that the frequency of the beat signal becomes a predetermined value, and the frequency of the laser is measured by measuring the frequency of the beat signal.
- Here, the repetition frequency frep and the CEO frequency fCEO may be measured, and at least one of the repetition frequency frep and the CEO frequency fCEO may be changed such that the frequency of the beat signal becomes the predetermined value.
- Furthermore, an absolute frequency v laser of the laser may be measured from measurement values of the repetition frequency frep and the CEO frequency fCEO, and the predetermined value or measured value of the beat signal.
- Also, the repetition frequency frep may be stabilized by phase synchronization with an frep reference frequency, which is generated for stabilization of the repetition frequency frep, and the CEO frequency fCEO may be stabilized by phase synchronization with an fCEO reference frequency, which is generated for stabilization of the CEO frequency fCEO. Also, at least one of the frep reference frequency and the fCEO reference frequency may be changed such that the frequency of the beat signal becomes the predetermined value.
- Also, the repetition frequency frep may be stabilized by phase synchronization with the frep reference frequency, which is generated for stabilization of the repetition frequency frep, and the CEO frequency fCEO may be measured and changed such that the frequency of the beat signal becomes the predetermined value to measure the oscillation frequency of the laser.
- The reference frequencies may be generated by using a frequency synthesizer.
- The present invention provides a laser frequency measurement device using an optical frequency comb, in which the frequency of a laser is measured by measuring a frequency of a beat signal generated by the interference between the optical frequency comb, used as the reference of measurement, and the laser to be measured. The laser frequency measurement device includes means for changing at least one of a repetition frequency frep and a CEO frequency fCEO of the optical frequency comb such that the frequency of the beat signal becomes a predetermined value, and means for measuring the frequency of the beat signal to measure the frequency of the laser.
- Here, the laser frequency measurement device may include means for measuring the repetition frequency frep, means for measuring the CEO frequency fCEO, and means for changing at least one of the repetition frequency frep and the CEO frequency fCEO such that the frequency of the beat signal becomes the predetermined value.
- Furthermore, the laser frequency measurement device may include means for measuring an absolute frequency v laser of the laser from measurement values of the repetition frequency frep and the CEO frequency fCEO, and the predetermined value or measured value of the beat signal.
- Also, the laser frequency measurement device may include means for stabilizing the repetition frequency frep by phase synchronization with an frep reference frequency, which is generated for stabilization of the repetition frequency frep, means for stabilizing the CEO frequency fCEO by phase synchronization with an fCEO reference frequency, which is generated for stabilization of the CEO frequency fCEO, and means for changing at least one of the frep reference frequency and the fCEO reference frequency such that the frequency of the beat signal becomes the predetermined value.
- Also, the laser frequency measurement device may include means for stabilizing the repetition frequency frep by phase synchronization with the frep reference frequency, which is generated for stabilization of the repetition frequency frep, means for measuring the CEO frequency fCEO, and means for changing the CEO frequency fCEO such that the frequency of the beat signal becomes the predetermined value.
- Also, the laser frequency measurement device may include means for generating the reference frequencies by using a frequency synthesizer.
- According to the present invention, when the beat frequency varies out of the predetermined range, stabilization control is performed by changing one or both of the repetition frequency frep and the CEO frequency fCEO of the optical frequency comb in the direction of compensating a variation of the beat frequency. Therefore, it is possible to measure the frequency of a laser that varies in a wide range, irrespective of limitations of a band of a band pass filter, which is used in measurement of the beat frequency. This configuration can achieve the measurement of the frequency of an inexpensive stable laser, which is often used in an industrial field.
- These and other novel features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments.
- The preferred embodiments will be described with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein:
-
FIG. 1 is a graph showing examples of spectra of an optical frequency comb and a laser; -
FIG. 2 is a graph showing an example of a spectrum of a beat frequency signal; -
FIG. 3 is a block diagram showing the configuration of a first embodiment of the present invention; -
FIG. 4 is a drawing of an example of the optical frequency comb used in the first embodiment; -
FIGS. 5A to 5C are graphs showing a spectrum of the beat frequency signal after passing through a band pass filter in the first embodiment; -
FIG. 6 is a block diagram showing the configuration of a second embodiment of the present invention; -
FIG. 7 is a block diagram showing the configuration of a third embodiment of the present invention; and -
FIG. 8 is a block diagram showing the configuration of a fourth embodiment of the present invention. - Embodiments of the present invention will be described below in detail with reference to the drawings. Note that, the present invention is not limited to descriptions of the below embodiments and practical examples. Components of the embodiments and the practical examples described below contain what is easily assumed by those skilled in the art, what is substantially the same, and what is in a so-called equivalent scope. Moreover, the components disclosed in the embodiments and the practical examples described below may be appropriately combined with each other or appropriately selectively used.
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FIG. 3 shows a first embodiment of a laser frequency measurement device using an optical frequency comb according to the present invention. - An
optical frequency comb 100 is stabilized by an frep control signal and an fCEO control signal from acontroller 110. Afrequency counter 120 measures a stabilized repetition frequency frep, and afrequency counter 130 measures a stabilized CEO frequency fCEO. A personal computer (PC) 140 receives measurement values thereof, and monitors the oscillation frequency of theoptical frequency comb 100. - A
photodetector 160 detects a beat signal generated by alaser 150 to be measured and theoptical frequency comb 100. Afrequency counter 170 measures the frequency of the beat signal, and thePC 140 receives a measurement value thereof. - Then, when the beat frequency exceeds a predetermined range, a frequency change command is sent to the
controller 110, so that thecontroller 110 sends the frep control signal and/or the fCEO control signal to theoptical frequency comb 100. - In the drawing, a
reference numeral 152 refers to a mirror, and areference numeral 154 refers to a half mirror. - An example of the
optical frequency comb 100 will be described with reference toFIG. 4 , which citesFIG. 1 of Patent Literature 1. In an opticalfrequency comb oscillator 10, first, light excited by an LD (laser diode) 12 generates in a ring resonator a laser that supports a plurality of longitudinal modes. Then, by adjusting a plane of polarization of the laser orbiting in the ring resonator with the use of polarizing elements (14, 15, and 16), such as a wave plate and a polarizing plate disposed in the ring resonator, phase synchronization among the plurality of longitudinal modes occurs, and a pulsed laser is generated. The frequency spectrum of the pulsed laser at this time is in the shape of a comb having a repetition frequency of frep. The repetition frequency frep can vary by varying the length of the resonator. Thus, in a technique ofFIG. 4 , the repetition frequency frepis changed by adjustment of an extension amount of anoptical fiber 11 by aPZT 13. On the other hand, the CEO frequency fCEO can vary by varying the excitation power, and hence is controlled by changing an injected current from a driver to theLD 12. - In the drawing, a
reference numeral 2 refers to a laser light source.Reference numerals Reference numerals Reference numerals Reference numerals Reference numerals Reference numerals Reference numerals Reference numerals Reference numerals reference numeral 52A refers to a non-linear optical medium. Areference numeral 53B refers to a mirror.Reference numerals reference numeral 55A refers to a band pass filter. Areference numeral 56A refers to a CEO frequency detector. Areference numeral 56B refers to a heterodyne detector. Areference numeral 56C refers to a repetition frequency detector. Areference numeral 60 refers to a CEO frequency stabilizer. Areference numeral 70 refers to a repetition frequency stabilizer. - In an optical frequency comb disclosed in Patent Literature 1, the
CEO frequency stabilizer 60 and therepetition frequency stabilizer 70 are used for stabilizing the frequencies. However in this embodiment, theCEO frequency stabilizer 60 and therepetition frequency stabilizer 70 are used in an opposite manner to change the frequencies. - In this embodiment, if the oscillation frequency of the
laser 150 varies and the beat frequency exceeds, or is likely to exceed, the measurable frequency range (in a case where a state ofFIG. 5A is changed to a state ofFIG. 5B , which schematically show spectra of the beat frequency signal), thePC 140 sends the oscillation frequency change command. In response to the command, thecontroller 110 sends the frep and/or fCEO control signals, to perform frequency control so as to compensate a variation of the beat frequency fB. - Taking a beat frequency that is generated by the interference between the optical frequency comb and the laser to be measured having the spectra of
FIG. 1 as an example, if the beat frequency fB increases due to increase in the frequency vlaser, the repetition frequency frep is increased or the CEO frequency fCEO is increased under control, so that the beat frequency fB is changed to be in the predetermined range, for example, returned to the center of the BPF. In the spectrum of the beat frequency, the state ofFIG. 5B is changed to a state ofFIG. 5C . - Measuring the repetition frequency frep and/or the CEO frequency fCEO changed at this time by the frequency counters 120 and 130 facilitates obtainment of the absolute frequencies of the
optical frequency comb 100, and therefore it is possible to measure the absolute frequency Vlaser of thelaser 150 to be measured without any problems. - To stabilize the oscillation frequencies of the optical frequency comb, there is a method by which the repetition frequency frep and/or the CEO frequency fCEO are synchronized in phase to frep and/or fCEO reference frequencies, respectively, each of which is generated in accordance with the repetition frequency frep and/or the CEO frequency fCEO. In a method for generating the reference frequencies in this case, a frequency synthesizer is used. An input of a highly accurate standard frequency of 10 MHz makes it possible to generate the reference frequencies with great accuracy. Thus, the optical frequency comb that is stabilized by phase synchronization with the reference frequencies constitutes the laser frequency measurement device with high accuracy.
FIG. 6 shows a second embodiment of the present invention for realizing this method. - Upon receiving the measurement value of the beat frequency signal, the
PC 140 sends a reference frequency change command tofrequency synthesizers 200 and/or 210, which generate the frep or fCEO reference frequencies respectively, so that the beat frequency is controlled to fall within the predetermined frequency range. How to change the repetition frequency frep and/or the CEO frequency fCEO in accordance with an actual change of the beat frequency fB is the same as that of the first embodiment. - According to the second embodiment, since the accuracy of frequency stabilization by the phase synchronization is much higher than the accuracy of frequency measurement by the frequency counters, it is possible to perform frequency measurement with higher accuracy than the first embodiment.
-
FIG. 7 shows a system configuration that more easily achieves accuracy sufficient for practical use, according to a third embodiment. - An error remaining in stabilization control of the repetition frequency frep is multiplied by the number “n” of orders thereof. Since “n” is a value of several millions, extremely high accuracy is required of stabilization of the repetition frequency frep. On the contrary, an effect of an error remaining in stabilization control of the CEO frequency fCEO is much smaller than accuracy required for stabilization of the repetition frequency frep. Accordingly, in this embodiment, the repetition frequency frep is stabilized by phase synchronization with the frep reference frequency having high accuracy, with the use of the
frequency synthesizer 200. On the other hand, the CEO frequency fCEO is controlled by sending a command signal from thePC 140 to anLD controller 300 so that the beat frequency fB measured by thefrequency counter 170 falls within the predetermined range. Thefrequency counter 130 measures the controlled CEO frequency fCEO. If the beat frequency fB varies due to a variation in the oscillation frequency of thelaser 150, the CEO frequency fCEO is changed so as to compensate a variation of the beat frequency fB. - This holds promise of an effect of extending a measureable range ±ΔfB of the beat frequency fB to a range from −ΔfCEO−ΔfB to +ΔfCEO+ΔfB, even with limitations of an fCEO oscillation frequency range of ±ΔfCEO.
- Note that, as a fourth embodiment shown in
FIG. 8 , a method may be combined in which thePC 140 commands thefrequency synthesizer 200 to generate the frep reference frequency, and to change the frep reference frequency, so that the beat frequency fB varies. The combination allows measurement of a variable laser frequency in a wide range, out of the range from −ΔfCEO−ΔfB to +ΔfCEO+ΔfB. - It should be apparent to those skilled in the art that the above-described embodiments are merely illustrative which represent the application of the principles of the present invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and the scope of the invention.
Claims (7)
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US11313930B1 (en) | 2020-11-13 | 2022-04-26 | Rohde & Schwarz Gmbh & Co. Kg | Alternation pulsed double resonance detection scheme for gapless detection in atomic vapor quantum sensors |
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CN113759172B (en) * | 2021-08-26 | 2022-06-21 | 上海交通大学 | Broadband and real-time microwave photon frequency measuring device and method based on microwave frequency comb |
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